1. Field of the Invention
The present invention relates to an LPP (laser produced plasma) type EUV (extreme ultra violet) light source apparatus for generating extreme ultra violet light to be used for exposure of semiconductor wafers or the like. Further, the present invention relates to a driver laser for applying light to a target in the LPP type EUV light source apparatus.
2. Description of a Related Art
Recently, as semiconductor processes become finer, the photolithography has been making rapid progress to finer fabrication, and, in the next generation, microfabrication of 100 nm to 70 nm, further, microfabrication of 50 nm or less will be required. For example, in order to fulfill the requirement for microfabrication of 50 nm or less, the development of exposure equipment with a combination of an EUV light source for generating ultra violet light of about 13 nm in wavelength and a reduced projection catoptric system is expected.
In such an EUV light source apparatus, generally, a short-pulse laser is used as a driving light source (driver), because the short-pulse laser is suitable for obtaining high CE (conversion efficiency from applied laser light to EUV light) in an LPP type EUV light source apparatus.
FIG. 10 is a schematic diagram showing a constitution of an oscillation amplification type laser to be used as a driver.
An oscillation amplification type laser 10 shown in FIG. 10 includes an oscillator 11 formed by a short-pulse CO2 laser and an amplifier 12 for amplifying a laser beam generated by the short-pulse CO2 laser. Here, in the case where the amplifier 12 has no light resonator, a laser system having such a constitution is referred to as “MOPA (Master Oscillator Power Amplifier) system”. The amplifier 12 has a discharging device that excites by discharge a CO2 laser gas containing carbon dioxide (CO2), nitrogen (N2), helium (He), and further containing hydrogen (H2), carbon monoxide (CO), xenon (Xe), etc. according to need.
In the case where a resonator is provided in the amplification stage unlike the amplifier 12 shown in FIG. 10, laser oscillation can be performed by the single amplification stage. A laser system having such a constitution is referred to as “MOPO (Master Oscillator Power Oscillator) system”.
The laser beam having energy “A” output from the oscillator 11 is amplified into a laser beam having desired energy “B” in the amplifier 12. The laser beam having energy “B” is collected through a laser beam propagation system or a lens, and applied to an EUV emission target material selected from among tin (Sn), xenon, etc.
In FIG. 10, only one stage of amplifier is provided for amplifying the laser energy “A” to laser energy “B”. However, when desired laser energy “B” is not obtained, plural stages of amplifiers may be used.
Next, a constitutional example of short-pulse CO2 laser as an oscillator will be described. In FIG. 5 of U.S. Pat. No. 6,697,408 discloses a constitution of a short-pulse RF (Radio Frequency) excitation CO2 laser. In this short-pulse RF-CO2 laser, highly repetitive operation of laser pulses can be performed to about 100 kHz. Practically, it is necessary to obtain EUV emission of 100 W class, however, when CE by the CO2 laser is estimated at 0.5% and propagation loss is estimated at 70%, the output required for the CO2 laser is about 60 kW. In order to achieve the output of 60 kW in the short-pulse laser, the repetition frequency of 50 kHz to 100 kHz is required when considering the durability etc. of optical elements and so on.
The reason is as below. Assuming that the output energy of the CO2 laser is Etotal, the repetition frequency of pulse oscillation is fi (i=1, 2, 3, . . . ), and the light energy of a single pulse is Epj (j=1, 2, 3, . . . ), there is a relationship as Etotal=f1×Ep1=f2×Ep2. Here, if Ep is larger, the damage on optical elements through which the laser beam is transmitted becomes also larger, and thereby, the optical elements deteriorate rapidly. Accordingly, Ep is desirably smaller. Therefore, to obtain the desired Etotal, Ep may be made smaller and the frequency fi may be made larger.
In order to realize such high repetition operation, an RF (Radio Frequency) excitation CO2 laser is suitably used. The reason is that, although there is a TEA (Transverse Excitation Atmospheric) CO2 laser as another pulsed CO2 laser, the operation at about 2 kHz is a limit in the current technology.
Referring to FIG. 5 of U.S. Pat. No. 6,697,408, the laser apparatus includes a multipass waveguide laser oscillator 400 and a multipass waveguide laser amplifier 400a. A resonator of the oscillator 400 is formed by total reflection mirrors 408 and 406. A Q-switch, an RF discharge unit, and a thin film polarizer (TFP) are provided between the mirrors. When the Q-switch is OFF, a laser beam travels back and forth between the mirror 408 and the mirror 406, and light intensity increases because of simulated emission at the time. When the Q-switch is turned ON at the time when the light intensity increases sufficiently, highly peaked short pulses are reflected in the TFP 404 and directed into the multipass waveguide laser amplifier 400a shown in the lower part of FIG. 5 via a mirror 409 and a half-wave plate. Then, the introduced light is amplified in an amplifier, and a laser beam is extracted to the outside. The laser having such a constitution is referred to as “Q-switched cavity-dumped laser”.
Further, related technologies are described in the following documents (1) to (5).    (1) Fumihiko Kannari, “Numerical Simulation on the Amplification of Picosecond Pulses in Multiatmosphere CO2 Laser Media”, The Review of Laser Engineering, No. 17, No. 2, pp. 45-59, Received on Aug. 18, 1988.    (2) A. Endoh et al., “Temperature control of multiline oscillation of a TEA CO2 laser”, Journal of Applied Physics, 50(8), August 1979, pp. 5176-5178.    (3) I. V. Pogorelsky et al., “Subnanosecond Multi-Gigawatt CO2 laser”, IEEE Journal of Quantum Electronics, Vol. 31, No. 3, March 1995, pp. 556-566.    (4) F. Rotermund et al., “Difference-frequency generation of intense femtosecond pulses in the mid-IR (4-12 μm) using HgGa2S4 and AgGaS2”, Optics Communications 185, 2000, pp. 177-183.    (5) V. Petrovet al., “Generation of high-power femtosecond light pulses at 1 kHz in the mid-infrared spectral range between 3 and 12 μm by second-order nonlinear processes in optical crystals”, Journal of Optics A: Pure and Applied Optics 3, 2001, R1-R19.
As in conventional technologies, in the RF-CO2 short-pulse laser using Q-switching, since the laser gas pressure is low (generally, 40 Torr to 100 Torr), broadening of each CO2 laser transition is not sufficient and the gain spectrum is modulated in a comb-like pattern. That is, in the case where the laser gas is at low pressure, as shown in FIG. 11, the oscillation spectrum of CO2 laser becomes not continuous but discrete. As a result, as disclosed in the document (1), a gain in the amplification process by simulated emission, saturation thereof and a pulse shape depend on a degree of matching between a spectrum of the laser pulse and a gain spectrum.
By the way, in the case where the output light of the oscillation stage laser is directed into the amplifier in the subsequent stage and amplified as in the above-mentioned MOPA or MOPO system laser, the output spectrum of the oscillation stage laser largely affects the amplification efficiency. For example, if the oscillation stage laser has a single spectrum, that is, light energy is concentrated within a specific narrow wavelength band, the gain becomes readily saturated in the amplification stage. In the case where such a single-spectrum laser beam is input to the amplification stage of the above-mentioned MOPA or MOPO system laser and amplified, since the amplifiable spectrum is limited to one, the amplification efficiency in the multi-spectrum (multi-line) amplifiable amplification stage becomes lower. That is, since only one spectrum contributes to amplification and the rest of the spectrums cannot be laser-oscillated and contribute to amplification, and thereby, a great part of gain is wasted. This means that high output is not obtained in the amplification in the amplification stage.